"Breeding-back" aims to restore or immitate extinct animals by selective breeding. This blog provides general information, the facts behind myths and news from various projects.

Monday, 5 February 2018

Biological basics I: The species concept - a complicated issue

Todays post
is on a rather theoretical topic, namely the definition of a species. It is
very basic, but a necessary requirement for other issues such as hybridization
or the subspecies concept, which are relevant for several units of
“breeding-back”. At first it might seem easy to define a species, but everybody
who has a basic deeper knowledge of biology will know that it is actually a
very tricky issue. There are several concepts and definitions of a species, and
none of them qualifies as the ideal and only one. This is why there is the
saying “in a room of n biologists there
are n+1 species definitions”.

The reason
for this problem is that the taxonomical system that classifies organism is an
artificial one made by us humans in order to work efficiently with the
diversity of living beings. Classical Linnean ranks, thought to us in school
and common to most people having a basic biological knowledge, are painfully
artificial and subjective which is why they are usually avoided in modern
phylogenetic systematics and cladistics. The basic biological entity that systematics
are based on are individuals (even here we have blurred lines, see animal
colonies; however, this blog deals only with vertebrates so let us ignore those
for now). The next level are populations, and above that, biological ranks begin
and so does subjectivity. Strictly spoken, a species is more of a hypothesis
than an entity. In this post, I want to outline the different approaches to how
define a species, their problems and also give some examples.

We want to
focus on vertebrates here. For other organismic groups, such as the asexually
reproducing bacteria, there are other approaches on how to define a species. The
most common species definition was coined by Ernst Mayr and is widely known as
the biological species concept. As “biologic” is rather generic, I refer to as
the Mayr’s definition in this post. It defines a group of individuals or
populations that actually or potentially reproduce and produce fully fertile
offspring. This concept works well in many cases, which is why it is widely
used. But there are also examples where it becomes problematic. One problem are
hybridization in the wild; many species have hybrid zones with closely related,
neighbouring species. Prominent examples are fire-bellied toads (Bombina), or Canis, where wolves hybridize with golden jackals in Eurasia1
and coyotes in North America, and so do the so-called pariah dogs2. A
special case of hybridization in the wild that makes the Mayer’s definition problematic
are the so-called ring species. Another example that is relevant for us are
American and European bison (Bisonbison and Bisonbonasus). Both are geographically
isolated because they dwell different continents, but when brought to the same
area they reproduce readily and produce fully fertile offspring. This is why some
authors have suggested listing them as one species, giving them subspecies
status (although both the European and American bison are further divided into
two subspecies themselves). However, most zoologists still consider both
different species. One reason is another level that is relevant for species
recognition, morphology. There is the so-called morphological concept or
morphospecies that differentiates species based on phenotypical traits. Palaeontologists
mostly have to work with morphospecies only due to a lack of genetic, ethologic
and ecologic data. The problem of morphospecies are intraspecific variation,
which can also lead to a morphologic overlap between closely related species,
as much as phenotypic plasticity. The ambiguity of the morphospecies concept
shows for example in the case of lion (Pantheraleo) and tiger (Pantheratigris). The
skeleton of a female tiger and a male lion are almost indistinguishable3.
However, apart from other morphological differences such as coat colour and
hair growth (mane), lions and tigers have different social behaviour, hunting behaviour
and despite an overlap in range do not reproduce in nature, which is why they
are considered separate species. This, as much as the bison example, shows that
in practice not just one species definition but often a mix of several levels
(genetic, morphologic, ethologic and ethologic) is used to determine and
differentiate species.

The third
relevant definition of a species is called the phylogenetic concept, which
deals with species that evolved through cladogenesis (that is the split of
evolutional lines; the direct evolution from one species to another without
cladogenesis is called anagenesis). In this case a species is a monophyletic
clade of one or more population that ends either through speciation (the
evolution of a new species) or extinction. This concept is only useful under
the frame of phylogenetic systematics or cladistics.

Having had
a look at what species actually are, we can dive deeper into the issues of
hybridization and subspecies, which is directly relevant for “breeding-back”
related topics concerning the wisent, quagga and others that have been covered
here on this blog.

1 comment:

Yes a complicated issue because life is a continuum and not an conglomerate of individuals and species isolate. It's hard because science has a need to isolate and identify in a micro prespective but life does not limit itself to tham perspective, it's from the macro one can understand there is no real reparation between nothing in this planet. It's just useful to judge like that from a learning posture prespective but it's far from beeing correct.

About this blog

This blog is on everything related to the so-called “breeding-back” of extinct animals: From the extinct animals themselves, over their often domestic descendants and dedomestication to news and facts about various breeding-back projects, reports and photos from my own breeding-back related trips. I try to have a balanced and fact-based approach to this subject and to dismantle many of the popular myths. Enjoy!

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About me

My major interest always have been extinct animals, from dinosaurs to Pleistocene megafauna and more recent extinctions. Besides that I am interested in evolution, genetics and ecology.
I am also an amateur animal artist, making drawings and models mostly of extinct animals.